1) take ALL between " and "
2) So you have ALL in $1. $1 is read only, so copy $1 to $x. And now, for one moment forget about all and work only on $x (this is thanx /e which "wraps an eval{...} around the replacement string and the evaluated result is substituted for the matched substring".
3)in $x replace all ' ' to '_'
4)put replaced $x into first s/from/to/
5)g - works globaly; e - eval the replacment string

While the explanation provided by YAPE::Regex::Explain will probably be inferior to that which a human well-versed in regexps could produce, the advantage in using the module is that an explanation can be produced without having access to the well-versed human. It is available to anyone with access to CPAN, and therefore can be quite useful to the learner. :-)

Another suggestion: if the whole regex is surrounded by a fully qualified (?xi-sm: ... ) block, then it's likely created by the qr// stringification, and not as presented. I would consider stripping that off and then explain the modes at the end along with any /g /e explanations.